The electrolytic production of chlorine and caustic by the electrolysis of brine has been well known for many years. Historically, diaphragm cells using a hydraulically-permeable asbestos diaphragm, vacuum-deposited onto foraminous steel cathodes, have been widely commercialized. Such diaphragm cells, employing permeable diaphragms, produce NaCl-containing NaOH catholytes because NaCl passes through the diaphragm from the anolyte to the catholyte. Such NaCl-containing caustic is generally of low caustic concentration and requires a de-salting process and extensive evaporation of water to obtain a low-salt, high concentration caustic for industrial purposes.
In recent years, the chlor-alkali industry has focused much of its attention on developing membrane cells to produce low-salt or salt-free, high concentration caustic in order to improve quality and avoid the costly de-salting and evaporation processes. Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated Na.sup.+ ions to be transported from the anolyte portion to the catholyte portions, while substantially preventing transport of Cl.sup.- ions. Such cells are operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium. The anodic reactions and cathodic reactions are not affected by the use of a membrane cell as opposed to the use of a diaphragm cell.
In addition to the caustic strength being important, two other criteria of the operating cell must also be considered for a complete energy view of the overall process. One is current efficiency, which is the ability of the membrane to prevent migration of the caustic produced at the cathode into the anode compartment; and the second is the voltage at which the cell operates, which is partly determined by the electrical resistance of the membrane. Power efficiency is often used as one term that considers both the current efficiency and cell voltage. It is defined as the product of the theoretical voltage, divided by the actual voltage, multiplied by the actual amount of caustic produced divided by the theoretical amount of caustic that could have been produced at a given current. Thus, it is apparent that power efficiency is reduced by higher cell voltage or by lower current efficiency. The membrane has a direct effect on both. The most common method of comparing cells is to express the operation as kilowatt hours (KWH) of power consumed per metric ton (mt) of product produced. This expression also considers both voltage, higher voltage increasing the quantity KWH, and current efficiency, lower efficiency decreasing the quantity of product produced (mt). Thus, the lower the value KWH/mt, the better the performance of the cell. It is apparent that optimization of a membrane for use in electrolytic chloralkali cells is a trade off between cell voltage which is reflected in membrane electrical resistance, current efficiency and caustic concentration.
It is well known (G. E. Munn, Nafion.RTM. Membranes--Factors Controlling Performance in the Electrolysis of Salt Solutions, The Electrochemical Society Meeting, October, 1977, Atlanta, Ga.) that the current efficiency of a chlor-alkali cell containing a membrane is determined primarily by the surface of the membrane contacting the catholyte. The current efficiency is dependent on the equivalent weight of the membrane in contact with the catholyte and the voltage is dependent on both the thickeness of the membrane and the equivalent weight of the membrane. The equivalent weight is the measure of the concentration of ion exchange functional groups in the polymer membrane and is simply the weight of the polymer in the acid form required to neutralize one equivalent of base. The above publication discloses that lower equivalent weights (eq. wts.) have lower electrical resistance (and thus lead to lower cell voltage), but that higher eq. wts. are required to obtain sufficient negative ion rejection and thus acceptable current efficiency. It is well known and discussed in the publication that voltage drop across the membrane is directly dependent on thickness; a thin film being desirable for minimum voltage drop. It thus follows that ideal membranes would be very thin films having higher eq. wts. (1500-2000 for sulfonic acids membranes of the prior art).
U.S. Pat. No. 3,909,378 teaches a method to take advantage of the increased current efficiency associated with high eq. wts. without absorbing the full voltage penalty associated with these materials. This patent teaches a composite membrane formed by laminating a thin, high eq. wt. film to a thicker, lower eq. wt. film. The thin, higher eq. wt. side of the film faces the catholyte in the cell thus resulting in current efficiency associated with the higher eq. wt. and voltage associated with the thin layer plus the minimal voltage of the lower eq. wt. layer. The patent further teaches that the eq. wts. of the polymers fall within the range of 1000-2000 or even greater and that the eq. wt. difference between the low and high eq. wt. portions of the composite film should be at least 250 and preferably 400. The patent teaches polymers having sulfonyl type ion exchange groups and that the structure linking these groups to the main polymer chain are not critical. The sulfonyl ion exchange groups, according to the patent may be the sulfonamide form or in the sulfonic acid form.
U.S. Pat. Nos. 3,784,399 and 4,085,071 teach formation of a barrier layer, facing the catholyte, on a single polymer film by reacting ammonia or N-substituted amines with one face of a sulfonyl functional polymer to form sulfonamide ion exchange sites. The main distinguishing feature of these patents from U.S. Pat. No. 3,909,378 is that the barrier layer facing the catholyte is introduced by chemical modification on a sinqle eq. wt. film rather than by lamination of a barrier film to a support film.
U.S. Pat. No. 4,151,053 also teaches having barrier layers on the catholyte face of membranes to achieve enhanced current efficiency without substantial voltage penalties. The main distinguishing feature of this patent and U.S. Pat. Nos. 3,090,378; 3,784,399 and 4,085,071 is that the barrier layer has carboxylic acid ion exchange groups of the general structure .about.OCF.sub.2 COOM where M is hydrogen; ammonium; quaternary ammonium, particularly quaternary ammonium having a molecular weight of 500 or less; and metallic atoms, particularly alkali or alkaline earth metals. This patent teaches and claims two methods of achieving the barrier layer. One is a uni-layer film wherein the eq. wt. of the cation exchange groups are uniform and on one surface, to a depth of at least about 100.ANG., the sulfonyl groups have been substantially chemically converted to carboxylic acid groups. The second method to achieve a barrier layer is to use a two-ply film in which a first film having a higher eq. wt. value and a second film having a lower eq. wt. value are combined. The first film having the higher eq. wt. is chemically converted from sulfonyl groups to all, or at least part, carboxylic acid groups and faces the catholyte in the cell. A preferred embodiment is where only a thin stratum on one side of the first film is converted to carboxylic acid and the opposite side is laminated to the second film. The thin stratum of at least 100.ANG. faces the catholyte in the cell. The patent teaches that each film of the composite membrane should have eq. wts. in the range of 1000 to 2000 and that the first film, the high eq. wt. film, should have an eq. wt. at least 150 higher than the second film.
All of the aforementioned patents use as starting materials sulfonyl containing fluoropolymers wherein the sulfonyl is generally contained on a pendant chain. The useful polymers and monomer precursors for these type materials are described in U.S. Pat. No. 3,282,875. In each patent the preferred sulfonyl containing fluoropolymer is described as derived, by polymerization, from the monomer ##STR1## disclosed in the above patent. The polymers are generally copolymers of the above monomer and tetrafluoroethylene. These copolymers have become well known in the art and are sold under the tradename of Nafion.RTM. by E. I. duPont Company. These materials are so well known and widely evaluated as membranes in chlor-alkali cells that the properties of these materials, such as useful eq. wt. ranges, water absorption and the like, have become accepted as the properties of sulfonic acid containing fluorocarbon polymers. In general, useful eq. wts. for these materials in chlor-alkali cells is not below about 1000 to 1100. Below these values water absorption increases dramatically and physical integrity falls sharply. For eq. wts. above about 1800-2000, electrical resistance becomes so great as to render the materials impractical in chlor-alkali cell use. Preferred eq. wt. ranges are from about 1100 to about 1500.
U.S. Pat. No. 4,065,366 teaches the use of single layer carboxylic acid membranes in chlor-alkali cells. This patent teaches useful equivalent weight ranges that vary from about 500 to about 2000; the lower range being significantly lower than that claimed for sulfonic acid membranes. The usefulness of these membranes in chlor-alkali cells is taught as being associated with concentration of functional group in membrane (eq. wt.), water absorption of the membrane and glass transition temperature of the polymer. The most preferable range for the concentration of the carboxylic acid group in the polymer is given as 1.1 to 1.7 meq./g of dry polymer (about 600 to about 900 eq. wt.). Excellent current efficiencies are obtained with these relatively low eq. wt. carboxylic acid polymers at high caustic concentrations (30-40%), but the voltages reported in the examples are relatively high for the thicknesses reported (200 microns) and the current density of the cells (20A/dm.sup.2).